Transmission sysotfi



, v May 14, 1940. Q M1THE| ET AL 2,200,559

TRANSMISSION k'SYSTEM Filed oct. 11, 195e 2 sheets-shea*b 1 1| 2215 .EQS 1| E l l ll ll Y @t Y Nv\ |v El lim H.. I l I l Il 1 Hru rb wmm, Nw NN Il lllllllwlll H H H H H H m QN I Il .|.A||.| Il... III

o. MITCHELL Nm/TO Hwmu/sr ATTORNEY May 14, 1940- D. MITCHELL ET AL TRANSMISSION SYSTEM Filed Oct. 11, 1938 2 Sheets-Sheet 2 ATTORNEY armas mausmssron sesam nom mheu'nouna Broos, and Hammamet, Milburn, N. J., assignors to merican Telephone and Telegraph Company, a corporation ci New York `Application October l1, 1938, Serial No. Zd 14 claims. (ci iis- 1.5)

This invention relates to a transmission system. and particularly to a method of and means for assuring secrecy in the transmission of messages.

An object of the invention is the transmission of messages in such a manner that intelligible reception of the messages by unauthorized persons will be rendered very diiiicult.

Another object of the invention is to obtain the mammm emciency in the use of communication circuits by decreasing the amount of idle time of such circuits.

In accordance with a spe'cic embodiment of the invention, a plurality of one-way speech channels are associated with a larger number of subscribers telephone circuits; for example. six channels may b associated with ten different subscribers circuits. Switching means are provided whereby an idle channel is selected for transmission of .each dierent speech portion of vocal interval duration occurring in the various subscribers circuits. Properlyfcontrolled apparatus is provided at the receiving end of the line for reassembling the speech fragments or, in other words, for putting the various messages back together again, this apparatus being conseparate control channel.

-trolled by distinctive tones transmitted over a.

Privacy is achieved by use'of an arrangement of the type comprehended by the invention in view of the fact Athat snatches of several dillerent conversations appear in a random manner on any one of the various speech channels. Further, the arrangement is effective as a "channel saver, a fact apparent from the opening sentence of a preceding paragraph wherein it is pointed out that the number of subscribers telephone circuits served by the speech channels is greater than the number of said channels. This makes it possible to take advantage of the now -wellknown fact that, during a so-called conversation' period on a four-wire circuit, onev side of the circuit is actually in use only about one-third of the time.

`A feature of the invention is switching means eiective when a speech fragment of vocal inter- -val-duration is applied to a speech channel, to vpreselect the.nex idle channel; thatis, the

vchannel lto which the speech fragment of vocal interval duration occurring next will be applied. Another feature of the invention is the use of tones, to control the apparatus provided for reassembling the various speech' fragments of vocal interval duration at the receiving end of the line.`

In' general the termivocal interval as used above means the duration oiI a succession of syllables no one of which is separated from the pre-` causing relay -operation, being considered parts of the silent interval. Hang-over time" is a term commonly used in connection with systems including voice-operated devices and means the period that a voice-operated device remains, due to its own characteristics, in operated position after the operating impulse has lbeen removed. Hang-over time insures that weak endings of syllables, not capable by themselves, of holding the relays in operated position, be not lost by clipping. and may be increased or decreased as the type of system demands.

Applicants feel assured that -the arrangement of the present invention is capable of functioning with a high degree of privacy with a hang-over time of about 150 milliseconds; it will be lmderstood, however, that this period is subject "to variation depending upon conditions encountered in diierent systems.

It will be understood, of course, that the duration of the vocal intervals themselves will vary in accordance with the characteristics'of the system and the nature of the communication.

A full understanding of the operation of the arrangement contemplated by the invention as example, a two-way radio telephone system. Five subscribers sets i2, I3, il, I5 and I6 are shown at the west station, each of which is provided -with a suitable two-way subscribers telephone circuit; similarly,- flve subscriberssets i1, 2 2, 23, 24 and 25, are shown atthe east station, each -of there sets likewise being provided with a suitable two-way subscrlber's telephone circuit.

The west and east terminals are connected by three west-east speech channels 26, `Z'Iand 42, one west-east control channel Il, threeeast-west speech channels 44, 65 and 46 and one east-west control channel dll.

The west terminal switching apparatus is represented by box 52 and the east terminal switching apparatus by box 53; these switching arrangements will be Adescribed in detail subsequently by reference to Fig. 2.

The telephone circuit of subscribers set l2 has associated with it three oscillators 54, 55 and 56, each of which sends out a distinctive tone of frequency f1, f2 and f3, respectively; similarly the telephone circuit of subscribers set I3 has associated therewith oscillators 5l, 'l2 and 13, the

telephone circuit of subscribers set -l4 has asy sociated therewith oscillators 1, 'l5 and 16, the telephone circuit of set I5 has associated therewith oscillators ll, 82 and 83, the telephone circuit of set I6 has associated therewithoscillators de, t5 and 86. In va lsimilar manner alt-fthe east terminal oscillators 8l, |024 and :1&3 aressociated with the circuit of set "l1, oscillators |04, IDS and les with the circuit of set22, oscillators Ml, H2 and H3 with the circuit of set 2Q, oscillators its, H5 and HS with the circuit fofaset I# and ,oscillators lll, l22 and 123 -with thegcircuit of set 2%.

Each of the oscillatorsprovided at the vwest terminal sends out a distinctive tone; each oscil. lator provided at the east terminal sends out -af tonethesame as that of the corresponding oscillator at .the west terminal, that is, the tone sent out by oscillator 81 is the same as that sent out by oscillator 54, the tone sent out by oscillator mi is the same. as that sent out by oscillator 55 and s0 on. Each control channel 42 and 4l is capable of transmitting a frequency band suiciently broad to include the fifteen distinctive tones f1 to fis inclusive. At such time as a speech fragment of vocal interval duration'originating in one of the subscribers telephone circuits is applied to one of the speech channels, a tone from one of the three oscillators associated with that telephone circuit is applied to the control channel associated with the group of speech channels, the particular tone applied depending upon which speech channel has been selected for transmission of the speech fragment.

The operation of the terminal switching arrangements will be described in detail subsequently with reference to Fig. 2; their operation will be only very briefly described at this point. Let us assume that a conversation is taking place between subscribers B (set i3) and B' (set 22) and that a speech fragment of vocal interval duration4 originating at set i3 has `ust been applied to speech channel 25. The switching arrangement 52 operates at this time to do' several things, among them being to select an idle west-east speech channel (let us say channel 2li) to which the next speech fragment of vocal interval duration originated by set i3, or

by any of the other west sets, will beapplied and to apply tone from oscillator 51 to control chan-v nel 3, the tone f4 produced by oscillator 51 being the distinctive tone identioed with channel 26' so'far as subscribers. sets I3. and 22 are concerned. This tone actuates proper control apparatus ofterminal arrangement 53 in order to cause the speech fragment of vocal interval duration which it accompanied to be applied to the receiving side of the telephone circuit of subscribers set 22.

. v'l'.he next speech fragment of vocal interval` duration originating at the west terminal will be applied to speech channel 21, this channel havner for east-west transmission, the speech frag;r

ments of vocal interval duration being applied ing been preselected as described in the above paragraph. We' will ssume that this speech fragment originates at subscribers set It, it being assumed that subscriber C ls conversing with subscriber C'. At such time as this speech fragment is applied to the channel, switching apparatus 52 .acts to preselect an idle speech channel for the next speech fragment of vocal interval duration and to apply tone from oscillator 'l5 to control channel 43, `the tone, f8, produced by oscillator l5 heilig the dtinctive tone identified with channel 21 so far as subscribers sets I4 and 2,3 are concerned. This tone causes operation of certain apparatus associated with east terminal switching arrangement 53 elective to apply the speech fragment of vocal intervalduration coming in over speech channel 2l to theA receive side of the telephone circuit of subscribers set 23.

Speech fragments of vocal interval duration, as they originate in any of the telephone circuits at the west terminal, are, therefore, placed on the particular west-eastl speech channel which yhappened to be last preselected; x10-two consecuoperating in synchronsm at each endof teline.

The arrangement functions in a slililai"A lll to speech channels 4l, 45 and 46 as preselected by switching apparatus 53 and the accompanying control tones being applied to control channel d1.

It will be clear from the above description that it will be very diflicult for unauthorized persons, who may attempt to listen in on the speech channels, to interpret the messages being transmitted thereover as there will occur, on anyone of the speech channels, snatches of several dierent conversations, these conversation parts appearing in a purely random manner. The idea of preselecting. the next idle channel at the time a speechfragment of vocal interval duration is applied to a channel is particularly valuable as it assures that two consecutive speech fragments of vocal interval duration will notl be applied to the same channel. Y

The arrangement is also effective as a channel saver. Thus, in` the system disclosed, only six one-Wayspeech channels and two control channels are utilized for the interconnection of ten subscribers. With -a larger number of subscribers it is possible to closely approach the condition vof having only one-third as many speech channels in one direction as there are subscribers and yet provide adequate facilities for uninterrupted transmission. Thus, substantial operating economies are obtained by taking advantage of the previously recognized fact that duringthe time two subscribers are connected over a four-wire circuit, one side of the circuit is actually in use only about one-third of the time.. This is to say, during sixty seconds of conversation time, one side of the line is in use twenty seconds, the other side is .in se twenty seconds, and both sides `are idle twenty seconds. Where many long and 'expensive circuits are in- Referring now to Fig. 2, a detailed showing is set forth of'a portion of the switching apparatus of one of vthe terminals, for example, the West terminal, the switching apparatus of which is represented in Fig, 1 by box 52. In'order to avoid confusing duplication of similar apparatus, only the switching apparatus associated with one subscribers telephone set and sumcient of the apparatus common thereto and to other telephone sets to completely illustrate the operation of the ,entire system is disclosed.

Subscribers set H 32, which corresponds, for example, to subscribers set H2 of Fig. 1,' `is coupled to transmitting line H33 and receiving line H 33 byl'transformer |35, network H35 serving to provide a proper balance in the well-known marmer. Transmitting line |33 may be connected, through operation of certain relays which will be referred to in detail subsequently,

f to any one or three west-east speech channels H32, M13 and HM; these speech channels correspond to speech channels 28, 21 and 32 of Fig. l. Receiving line H 33 may be connected, through operation of certain relays which will be referred to in detail subsequently, to any one of three east-west speech channels H35, H86 and H31; these. speech channels correspond to speech channels 33, l5 and 36 of Fig. 1.

The major portion of the switching apparatus may be separated into three groups, one group being associated with each of the west-east transmitting channels. For example, it will be apparent from the subsequent description that relays |52, H53, H58, H55, H56, H51, |12, H13 and H15, oscillator and filter H16 are associated, so to speak, with speech channel |32, that relays H82, H83, H84, H85, |86, |81, ,282, 283 and Zilli,` oscillator 205 and lter 206 are associated `withspeech channel H 43 and that relays 2| 2, 213, 2M, 215, 2|6, 2H?, 232, 233 and 233, oscillatorl 235 and filter 236 are associated with speech channel H44.

Speech energy emanating lfrom subscribers set |32 follows two parallel paths, one path over transmitting line H33, through delay network 242 and out over one of the three west-east speech channels |32, H43 or I 46, depending upon which group of relays is operated, and the other path through amplifier-detector 2113 to the v various relay circuits.

In general, selection of the west-east speech channel is dependent upon the position of brush 255 of rotary selector switchZt; operation of this selector switch when a speech fragment of vocal interval duration is applied to a channel is eiective to preseleot the next speech channel in a manner evident from subsequent description. The switch may be of the type disclosed in O. F. Forsberg et al. Patent 1,472,465 ,and comprises two banks 231 and 252, two brushes cooperating therewith and a stepping magnet 253. It will be understood that the two brushes are mechanically coupled so that energization of stepping magnet 253 causes rotation of both lbrushes 235'and 258. vSwitch 236 is common to all of thesubscribers telephone circuits of the west terminal.

For purposes of further illustration of the invention, the operation of the circuits of Fig. 2 will now be described in detail. In the condition illustrated, relays H53 and H54` are operatedy lbreak contact of-relay |86, winding of relay I 5B to ground 2111 and winding of relay |53 to ground 215. Assuming now that subscriber A speaks, speech energy originating at subscribers set |32 is transmitted through transformer 35 into transmitting line |33. As the energy is delayed somewhat in its passage through delay device 232, let us rst see what operations result from passage of a part of the energy through amplifier-detector 253.

In view of the fact lthat relay 153 is operated, we have a completed path, for the energy emanating from the `amp'lier-detector, through conductors 216 and 211, which results in the completion of obvious energizing paths for relays H52, H55, H56, H51, H12, H13 and H13 all of which relays operate.

Operation of relay H13 connects, through its two make contacts, transmitting line |33 to westeast speech channel |32 so that the speech energy after traversing delay device 232 is sent out over speech channel H 32; operation of relay H13 connects through its two make contacts, the output of oscillator H15 through lter H16 to westeast control channel 282 whereby control tone f1 is applied to the control channel at the same time the speech energy is applied to the speech channel. Filter H16y is designed to pass only a small band of frequencies, including the tone frequency f1, and prevents the transmission of transients with the control tone.

Operation of relay |55 completes a locking path over which relays |53 and |56 are held operated, this path being traced from battery 283, make contact of relay |53, make contact of relay I55, winding of relay 154| to ground 213 and winding of relay |53 to ground 215.

Operation of relay |52 is effective to remove battery 284 from the make contactof relay 285: Relays 285, 286 and 281 are associated with the switching circuits of a second subscriber and correspond respectively to relays |53, |83and 2 I3 of the circuits associated' with subscrbers set |32; relays 288, 289 and 290 are also associated with the circuits of the second subscriber and correspond respectively to relays |52, H 82 and 2|2. In the instance of relays 285, 286 and 281,

` however, a connection to battery is not obtained directly through a contact of the respective reonly if said associated relay be in unoperated position. This arrangement is provided so that even if two subscribers were to start talking at the same instant there would be no chance of the speech of both being placed on the same channel. For example, with the circuits in the condition just described there is no chance of a second subscriber breaking in on channel |42 with subscriber A as operation of relay |52 has removed battery from the make .contact of relay 285 whereupon the holding circuit for relays in the other subscribers circuit corresponding to relays H 53 and H58 in the circuit of subscriber A cannot be established. In the instance of the switching circuits of the third subscriber, the battery supply path also includes the break contacts of relays 288, 289 and 290 of the second subscribers circuit; in the instance of subsequent subscribers circuits the path includes the break Contact of a'corresponding relay of the preceding circuit. Thus, for example, ifV the second sub-V scriber (B) and the fourth subscriber (D) were,

to start talking simultaneously, subscriber' ,A

being idle, subscriber B would be put on the next idle channel and connection of subscriber D thereto at the same time would be prevented due to the break in the battery supply path resulting from operation of relay 268. l

Operation of relay |56 causes a break in the path connecting certain relays of the group associated with channel |43 to channel selector brush 245; operation of relay |51 causes a similar break in the path connecting certain relays of the group associated withchannel |44 to the channel selector brush. This is done in order to prevent a possible operation of control relays |33 and |84 or 2|3 and 2|4 at this time should the selector switch be rotated. l

Operation of relay |12 completes an energizing path foi` stepping magnet 253, this path being traced from battery 292, make contact of relay H, conductor 293, third contact of switch bank y 252, brush 254, winding of magnet 253 to ground 234. ABrush 254 is therefore stepped around to the second contactY of bank 252 where it comes to rest. Had this contact been busy," i. e., had battery been connected thereto due to operation of relay 202 or a corresponding relay of another circuit, brush 254 would, of course, continue its rotation until it reached an idle contact of bank 252.

As brushes 254 and 245 are mechanically coupled, rotation of brush 254 to the second `contact of bank 252 causes a corresponding rotation of brush 245 to the second contact of bank 241. This rotation of brush 245 is effective to preselect channel |43 so that the next speech fragment of vocal interval duration emanating from any of the subscribers telephone circuits of the west terminal is placed thereon. This preselection is accomplished by setting up an energizing path for those relays associated with other subscribers sets which correspond to relays |83 and |64 associated with subscribers set |32. In the inn stance of relays |83 and |84, this energizing path isprepared but not completed as relay |56 is now in operated condition; the path when completed, i. e., when relay |56 is deenergized, is

'traced from battery 212, brush 245, second con' tact of bank 214, conductor 295, break contact l,of relay 2|6, break contact of relay |56, winding of relay .|34 to ground 296 and winding of relay m3 to ground 291. With this condition, therefore, the next speech fragment of vocal interval duration occurring in any of the circuits is placed on channel |43; should subscriber A cease talking long enough -forthe relays associated with channel |42 to restore to normal before any of the other west subscribers start talking, relays |83 and |B4Wi1l operate over the path described above and should he then resume speaking, the

resulting speech fragment will be placed on channel |43. l

Although battery 212 is removed from c onductor 213 as soon as brush 245 leaves the third contact of bank 24|.. relays |53 and |54 remain in operated position due to the locking circuit through the make contact relay of |55.

Returning now to consideration of conditions on channel |42, speech originatingk at subscribers set |32 is being transmitted `thereover .and a control tone of frequency f1 is being transmitted over control channel 262. Let us now consider the effect of these signals at the receiving end of the system. As the receiving apparatus of the east terminal has not been illustrated in Fig. 2,

let us refer instead to the receiving arrangement of the west terminal and assume that speech is to amplifier-detector 3|4 and, after rectification thereby, causes operation of relay 3|5.

Operation of relay 3|5 connects, through its make contacts, speech channel |45 to receiving line |34 so that the incomingl speech is transmitted into the receiving side' of the telephone circuit of subscribers set |32. A part of this received speech energy passes through amplifierdetector 322 and, after rectification thereby, causes operation of relay 3I6 and disables the transmitting line |33.

Considering again the transmitting circuit of Fig. 2, as soon as subscriber A pauses in his speech an appreciable period and speech energy is no longer passing through amplifier-detector 243, the relays associated with speech channel |42, that is, relays |52, |63, |54, |55, |56, |51, |12, |13 and |14 restore to unoperated position. As soon as relay |56 drops back to unoperated position, relays |83 and |184 voperate over an energizing vpath traced from battery 212, brush 245, second contact of bank 241; conductor -2535, break'contacts of relays 2|6 and |56, winding of relay |84 to groundV 256 and winding of relay |83 to ground 291. As corresponding relays of other groups are, as pointed out above, also in operated position the nextspeech fragment of vocal interval duration occurring in any of the telephone circuits of the terminal will be placed on speech channel |43 and control tone of frequency f2 or corresponding frequency if another subscriber talks will be sent out over control channel` 282.

Speech channel |44 and control tone f3 are preselected in a similar manner when brush 255 is rotated to the first contactV of bank 241 thereby connecting battery 212 to conductor 326 and setting up energizing paths for relays 2|3 and It will be understood, of course, that the other subscribers' ysets of the terminal are. provided with switching arrangements similar to that illustrated by Fig. 2. It will be understood further. that the number of channels (both4 speech and control) in the system described is merely by way of example; more channels or less may, of course, be provided depending upon the trafficdemands. y I

While a certain vspecific embodiment of the invention has been selected for illustration and detailed description, the, invention is not, of course, limited in its application to this embodiment. The embodiment described should be looked upon as illustrative of the invention and not as restrictive thereof.

What is claimed -is:

l. In a speech transmission system including a plurality of transmitters, an equal number of receivers, a plurality of transmissionchannels between said transmitters and said receivers, and means for impressing speech current produced byl said transmitters on said channels, the method of maintaining secrecy during the transmission of speech from said transmitters to said receivers which comprises impressing speech on said respective channels in the form of individual speech fragments each of a duration equal to a vocal interval and preselecting an idle channel for transmission of the nextv succeeding speech fragment each time a speech fragment of. vocal interval duration Iis impressed on a channel.

2. In a speech transmission system including 75 a plurality of transmitters, an equal number of receivers, and a plurality of transmission channels between said transmitters and Asaid receivers, the method of maintaining secrecy during the transmission of speech from said transmitters to said receivers which comprises impressing each successive speech fragment of a duration equal to a vocal interval produced by said transmitters on a different one of said channels, and preselecting an idle channel for the transmission of the next succeeding speech fragment of vocal interval duration each time a speech fragment .of such duration is applied to a channel.

3. In a speech transmission system, a plurality of transmitters, an equal number of receivers, a plurality of transmission channels between said transmitters and said receivers, means for applying a speech fragment of a duration equal to a vocal interval produced by one of said transmitters to a selected one of said channels, and means operating upon application of a speech fragment of vocal interval duration to one of said channels for preselecting an idle channel for application of the next succeeding speech fragment of such duration.

4. In a speech transmission system, a plurality of transmitters, a plurality of receivers, a plurality of transmission channels between said transmitters and said receivers, the number of said channels being less than that of said transmitters and said receivers, means effective upon production of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to a selected one of said channels, means for maintaining said connection during the duration of said speech fragment and means effective upon application of a speech fragment of vocal interval duration to one of said channels for preselecting an idle channel for application of the next succeeding speech fragment of such duration.

5. In a speech transmission system, a plurality of transmitters, a plurality of receivers, a plurality of transmission channels between said transmitters and said receivers, means effective upon production of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to a selected one of said channels, means for maintaining said connection during the duration of said speech fragment, means for preventing the connection of the output of another transmitter to said selected channel during the duration of said speech fragment, and means effective upon application of a speech fragment of vocal interval duration to one of said channels for preselecting an idle channel for application of the next succeeding speech fragment of such duration.

6. In a speech transmission system including an east terminal and a west terminal, a plurality of subscribers telephone circuits at each of said terminals, a plurality of transmission channels between said terminals, the number of said channels being less than that of said telephone circuits, aplurality of groups of relays associated with each of said telephone circuits, the number of said groups being equal to that of said channels, one group being associated with each channel, means eective upon occurrence of a speech fragment of a duration equal to a vocal interval in one of said telephone circuits for connecting the output thereof to a selected one of said channels, and means operating upon application of said speech fragment to said channel for causing operation of certain relays in all of said groups of relays associated with a second of said channels which is then idle, operation of said relays being eifective to preselect said second channel for application of the next succeeding speech fragment of vocal interval duration.

7. In a speech transmission system including a pair' of separated terminals, a plurality of speech transmitting sets at each of said terminals, 'a plurality of transmission channels between said terminals, the number of said channels being less than that of said sets, switching means for so interchanging the connections between said speech transmitting sets and said transmission channels that each successive speech fragment of a duration equal to a vocal interval produced by any of said sets at one of said terminals is placed on a diierent one of said channels, said switching means operating each time a speech fragment of vocal interval duration is applied to a channel to preselect an idle channel for application of the next succeeding speech fragment of such duration produced by any of said sets atv said terminal.

8. In a speech transmission system including an east terminal and a west terminal, a plurality ofy speech transmitting sets at said east terminal, a plurality of speech receiving sets at said west terminal, a plurality of speech transmission channels between said terminals, a plurality of sources of control current of different frequencies at said east terminal, a control toneA transmission channel between salid. terminals,

switching means at said east terminal for so changing the connections between said speech transmitting sets and said speech transmission channels that each successive speech fragment of a duration equal to a vocal interval produced by any of said transmitting sets is placed on a different channel, a plurality of frequency responsive switching devices at said west terminal each of which is responsive to the characteristic frequency of one of said sources of control current at the east terminal, each of said frequency responsive devices being associated with a respective one oflsaid speech receiving sets and being effective when operated to connect said receiving set to a predetermined one of said speech transmission channels, said switching means at said east terminal being effective each time a speech fragment of vocal interval duration is connected to a speech transmission channel to preselect an idle speech transmission channel for application of the next succeeding speech fragment of such duration produced by one of said speech transmitting sets and to connect one of said control current sources to said control tone transmission channel.

9. In a speech transmission systeml a plurality of transmitters, an equal number of receivers, a lesser number of speech transmission channels between said transmitters and said receivers, a signal transmission path, a plurality of distinctive frequency sources associated with each of said transmitters, the number of said sources associated with each transmitter being equal to the number of said channels, means for applying a speech fragment of a duration equal to a vocal interval produced by one of said transmitters to a selected one of said channels. means operating upon application of a speech fragment of vocal interval duration to one of said channels for preselecting an idle channel for application of the next succeeding speech fragment of such duration, and means effective during application of a speech fragmentl of vocal interval `duration to a channel for applying the tone produced by a selected one of the sources associated with that transmitter producing said speech fragment to said signal transmission path, the selection of said source being dependent upon which speech channel is selected for application of said speech fragment.

' 10. In a speech transmission system including an east terminal and a west terminal, a plurality of transmitters at the east terminal, an equal number of receivers at the west terminal, each of said receivers being adapted for connection with one of said transmitters, a plurality of speech transmission channels between said terminals, the number of Vsaid channels being less than the number of transmitters and receivers, means at the east terminal for applying a speech fragment of a duration equal to a. vocal interval produced by one of said transmitters' to a selected one of said channels, means at the west terminal for connecting the receiver adapted to be connected to said speech fragment producing transmitter to said selected channel, and means at the east terminal operating upon application of -a speech fragment of vocal interval duration to a channel for preselecting an idle channel for application of the next succeeding speech fragment of such duration produced by any of said transmitters.

11. In a speech transmission system including an east terminal and a west terminal, a plurality of transmitters at the east terminal, 4a plurality. of receivers at the west terminal, each of said receivers being adapted to be connected with one of said transmitters, a plurality of speech channels between said terminals, the number of said speech channels being less than the number of said transmitters and receivers, a control tone channel between said terminals, a plurality of distinctive frequency tone sources associated with each of said transmitters, the

number of said sources associated with each transmitter being the same as the number of said speech channels, means at the east terminal for applying a speech fragment of a duration equal to a vocal interval produced by one of said transmittersto a selected one of said speech channels and for applying a tone from one of the sources associated with the speech fragment producing transmitter to said control tone channel, means at the west terminal operating in accordance with the frequency of said applied tone for connecting the receiver adapted to be connected to said speech fragment producingv transmitter to-said selected speech channel, and means at the east terminal operating upon application of a speech fragment of vocal interval duration'to a speech channel for preselecting an idle speech channel for transmission of the next succeedingspeech fragment of such duration produced by any 'of said transmitters.V

12. In aspeech transmission system, a plurality of transmitters, a plurality of receivers, a plurality of transmission channels between said transmitters and said receivers, means effective upon prodmtion of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to a selected one of said channels, means for preventing the connection of the output lof a second transmitter to said channel while said first-mentioned transmitter output is connected thereto, and means effective upon application ,of a speech fragment of vocal interval duration to one of said channels for preselecting an idle channel for application of the next succeeding speech fragment of such duration.

13. In a speech transmission system including an east terminal and a west terminal, a plurality of transmitters at the east terminal, a plurality of receivers at the west terminal, a plurality of speech transmission channels between said terminals, a signal transmission channel between said terminals, means effective upon production of a speech fragment of a duration equal to a vocal interval by one of said transmitters for connecting the output thereof to a selected one of said speech channels, means for applying a distinctive signal tone to said signal channel effective to connect a selected one -of said receivers to that speech channel to which said speech fragment producing transmitter is con-- nected, and means for preselecting an idle Speech channel, said two last-mentioned means also being effective upon production of said speech fragment by said one transmitter.

14. In a speech transmission system including an east terminal and a west 'terminaL a plurality of subscribers telephonecircut's at each of said terminals, a plurality of transmission channels between said terminals, a plurality of groups of relays 'associated with each of said telephone circuits, each of said groups being associated with one of saidchannels in the sense that connection of a channel toa telephone circuit is dependent upon operation of a relay in the respective group of relays associated with said channel, means effective upon occurrence of a speech fragment of a duration equal to a vocal interval in ation of the relays in said last-mentioned-group which correspond to those relays in the other corresponding groups which are operated, said last-mentioned` means being effective during duration of said speech fragment, operation of said relays in 'said groups associated with said second channel being eective to preselect said second channel for application of the n'ext sucv ceedig'speech fragment of vocal interval duration.

DORllill MITCHELL HARRY NYQU'IST. 

